Studies on the structure of macromolecules in cancer cells are essential for defining the biochemical lesion(s) distinguishing cancer cells from normal cells. The present research proposal concerns 3 related topics: (1) A methodological investigation on postlabeling procedures for the sequence analysis of nonradioactive RNA; (2) Sequence analysis of specific 5-methylcytidine-rich tRNAs from normal and cancerous tissues; (3) Studies on the effects of an anticancer drug, 5-azacytidine, on RNA and DNA. Comments: (1) Structural analysis of small amounts of nonradioactive nucleic acids requires incorporation of radioactive label by postlabeling techniques. Methods to be developed will entail a combination of 3H- and 32P-labeling, digestion with nucleases, and chromatographic and electrophoretic separations. (2) We wish to determine the sequences of tRNAs that are rich in 5-methylcytidine, because 5-azacytidine was found in our laboratory to inhibit specifically the biosynthesis of this methylated compound in mammalian tRNA. (3) Preparation of nucleic acids deficient in a single modified constituent such as 5-methylcytosine may enable us to define the role(s) of this constituent. In vivo studies are planned to compare the effects of 5-azacytidine on tRNA in various normal and cancerous tissues and on DNA in rapidly growing normal and cancerous cells. In vitro studies will be aimed at elucidating the mechanism of inhibition of nucleic acid methylation and at identifying the biological functions of 5-methylcytidine in nucleic acids.